The present invention relates generally to a circuit for controlling thermal head elements of a thermal printing apparatus which modulates half-tone data of a video signal in a pulse width modulation (PWM) form and controls thermal head elements in a PWM mode. In a full color printer having a thermal head, a driving circuit for the thermal head controls heat generation of the thermal head elements through a pulse-width modulated signal (abbreviated as PWM signal) according to received half-tone data to provide different signals corresponding to the color densities.
Two methods have been adapted which drive the thermal head elements using such PWM signals.
One method is exemplified by the examined Japanese Pat. Publication JP No. 57-14315 published on Mar. 24, 1982.
In the above-identified Japanese Patent, a video image signal of an analog form is received by an A/D converter which converts the analog video signal into four-bit digital data in synchronization with a reference clock derived from a controller. A memory stores the digitally converted four-bit half tone-data into a predetermined memory area in response to a write command from the controller. The memory supplies the stored four-bit half-tone data to a shift register connected to the thermal head in response to the read signal from the controller. The data transfer from the memory to the shift register is carried out in such a way that respective most significant bit data (n data of bits "1" and "0") from the data for one horizontal line to be printed, i.e., the fourth bit of each data are at first serially supplied to the shift register. Upon the completion of the head driving based on the most significant bit of each of the n data for the line, respective second-most significant bit of each of the n-data (third-bit data) are then serially supplied thereto. In this way, the data transfer for one horizontal line is completed when respective the least significant bit of each of the data for the line are serially supplied thereto.
The controller outputs the reference clock to the A/D converter as described above and outputs a strobe signal to a latch-and-drive circuit. The latch-and-drive circuit latches the registered contents of the shift register for a duration corresponding to a pulse-width of the strobe signal, and supplies power to any head elements of the thermal head element group which correspond to bit portions of the register in which the bits "1" are registered. On the other hand, the latch-and-drive circuit does not supply the power to any other head elements which correspond to bit portions of the register in which the bits "0" are registered.
Each pulse-width of the above-described strobe signal is different depending on a weight of each bit of data. That is to say, the pulse-width of the strobe signal is 8d when the fourth-bit data is "1", 4d when the third-bit data is "1", 2d when the second-bit data is "1", and d when the first-bit date is "1". In this way, a signal corresponding to the color density of 16 tones can be provided with the sequential combination of the different pulsewidth strobe signals.
The other method is exemplified by the non-examined Japanese Patent Publication JOP No. 51-123511 published on Oct. 28, 1976.
In the above-identified latter Japanese Patent, the memory sequentially supplies the four-bit half tone data for one horizontal line to a comparator. In addition, a tone counter sequentially outputs sixteen reference tone signals for a first tone up to a sixteenth tone to the comparator. The comparator first compares each one of the four-bit tone data A in one horizontal line (n pixels), n four-bit data derived from the memory with the first reference tone signal B for the first tone derived from the tone counter. If A.gtoreq.B, the comparator produces a "1" logic signal. If A&lt;B, the comparator produces a "0" logic signal. Either of those output signals is transmitted from the comparator to the shift register. Next, the four-bit tone data A in the corresponding horizontal line (n pixels) derived from the memory is again outputted to the comparator. The comparator, then, compares the tone data A with the reference tone signal B for the second tone derived from the tone counter. Either of the two output signals indicating the comparison results is then transmitted from the comparator to the shift register. The above-described comparing operation is repeated up to the sixteenth tone. The controller outputs the strobe signal having a relatively narrow pulse-width to the latch-and-drive circuit upon completion of the data transfer of the output signals from the comparator to the shift register. The latch-and-drive circuit latches the data registered in the shift register in response to a rising edge of the strobe signal and continues the latch of data until the following strobe signal rises. The latch-and-drive circuit continues to supply the power to the head elements if the next incoming data from the comparator indicates "1". Consequently, signals corresponding to color densities in sixteen tones can be provided.
However, the two methods disclosed into the two above-identified Japanese Patent Application documents have the respective drawbacks described below.
In the former method, since the amount of transferred data for each horizontal line is expressed as 4.times.n and the strobe signal is modulated in the pulse-width modulation mode depending on the weight of each bit, the drive pulse for the thermal head is formed in a discontinuous PWM signal, in which one drive pulse is separated in time from the other drive pulse in cases where the data of "1 " are separately present. Therefore, the characteristics of the head elements are such that the drive pulse having the width d is activated after heat caused by the drive pulse having the width of 8d is almost dissipated. In addition, heat generation becomes reduced as compared with the case where the consecutive drive pulses having a continuous pulse-width of 9d activate the corresponding head elements. Hence, the appropriate, distortion-free thermal characteristics cannot be achieved.
On the other hand, in the latter method, the heat generation corresponding to each pulse-width of the drive pulses can be achieved since consecutive drive pulses are formed in the continuous PWM signal. Therefore, the appropriate distortion-free thermal characteristic can be achieved. However, the amount of transferred printing data for each horizontal line to the thermal heads is expressed as 16.times.n, which is considerably greater than that (4.times.n) in the former method. Therefor, if the number of tones is doubled, the amount of transferred data is doubled and accordingly the time required to print the tone data becomes doubled.